Hybrid breeding is an effective way of improving the production of crops. Compared to conventional plants, hybrids often exhibit heterosis, and usually have a significantly increased yield, better resistance, and wider adaptability. In addition, hybrid breeding is less time-consuming and has a shorter breeding cycle than conventional breeding. Therefore, hybrid breeding has become a major approach in the breeding of many crops.
An efficient male sterile line is the key factor in hybrid breeding. The male sterile line, which cannot produce effective male gametes is used as a maternal line to be pollinated by a paternal line. The following factors should be considered during the selection and generation of male sterile lines:
1. Hybrid vigor with other lines: the male sterile line can be crossed with other male-fertile lines to produce hybrids with a better combination of traits;
2. The reproduction of the male sterile line: the sterile line can restore fertility to self-maintain under certain conditions;
3. The efficiency of the reproduction and hybrid seed production using the male sterile line: a good sterile line should be easy to cross and lead to efficient hybrid seed production.
Male sterility can be either cytoplasmic or nuclear. Current hybrid rice utilizes the combination of both types of male sterility. Cytoplasmic male sterility (CMS) is caused by mutations in extranuclear genes and shows maternal inheritance. Manifestation of male sterility in CMS lines may be controlled through the interaction between cytoplasmic and nuclear factors. The widely used three-line method in hybrid rice breeding involves a male sterile line, a restorer line and a maintainer line. The three-line method requires specific restorer lines, which are generated through a complex process and greatly limits the utilization of heterosis among different varieties. By contrast, a two-line method utilizes a male sterile line, in which the sterility is controlled by a nuclear gene and the fertility can be restored under specific growing conditions, and therefore combines the restorer line and the maintainer line into one line. Compared with three-line method, two-line method has greatly simplified the hybrid seed production process by eliminating the demand of maintainer lines and significantly expanded the usage of male sterility hybrid breeding. However, there are also constraints in the utilization of two-line hybrid breeding method. The male sterile line need to switch fertility between ON and OFF under different conditions. It has to remain male sterile for hybrid seed production but be fertile to reproduce itself when the conditions change, in order to maintain the sterile system. The widely used male sterile lines in two-line method are mostly photo-thermo-sensitive sterile (PTGMS), and their fertility is influenced by temperature and light. Therefore, the instability of environment may result in the instability of the fertility of sterile lines, leading to either self-breeding and reduced purity of the hybrid seed, thereby increasing the risk of seed production. Furthermore, the methodology used for selection and generation of sterile lines for two-line method is very limited. For example, there are hardly any male sterile lines suitable for two-line method in Oryza japonica rice, restricting wide use of rice variety resources.
To bypass the problems existing in the current methods of hybrid rice breeding, such as the stability of the sterile line, the limitation of hybrid variety resources, the complexity in seed production and the high cost of seed production etc., a new hybrid breeding technique that can fully utilize male sterility controlled by recessive nuclear genes to construct stable sterile lines that are not affected by environmental changes to eliminate the potential risk in seed production is under development. Meanwhile, the recessive nuclear sterility gene is suitable for vast majority of crop varieties to improve heterosis utilization. Embodiments of the present disclosure provide a gene regulating plant fertility, the mutation of which results in male sterility and the sterility is stable and not influenced by environment and may be reversed through introduction of the wild-type gene into plants. The gene and the sterile line generated by the gene mutation provide necessary components for a new hybrid breeding system.